1. Field of the Invention
The present invention relates to an anamorphic converter suitable for a film camera, a television camera, a video camera or the like which is disposed on an image side of an imaging optical system in order to convert an aspect ratio to photograph an image having an aspect ratio different from that of an image pickup element, a lens device using the same, and an image pickup device such as a film camera, a television camera, or a video camera using the same.
2. Related Background Art
As a technique for converting an aspect ratio of an image to record and reproduce the resultant image, up to this time, various techniques have been proposed. In particular, for use in a motion picture, a system in which an image is optically compressed horizontally using an anamorphic lens to be photographed on a film, and during the reproduction, the image on the film is optically horizontally magnified to be projected using an anamorphic lens as well is generally used as a system for recording and reproducing an image in compliance with the CinemaScope form having an aspect ratio of 2.35:1. As an anamorphic converter, a large number of front converters each mounted on a side of an object of an imaging optical system were proposed (refer to Japanese Patent Application Laid-Open Nos. 48-24048, 2-13916, 3-25407, 5-188271, 5-188272, 6-82691 and Japanese Patent No. 2,817,074 for example).
In addition, a rear converter mounted to an image side of an imaging optical system was proposed (refer to Japanese Patent No. 3,021,985 for example).
In recent years, promotion of high image quality of the video technique has progressed, and a digital cinema system for making a film of a scene with an HDTV (High Definition Television) system is in the progress of being popularized. In a digital cinema system, an image pickup element having an aspect ratio of 16:9 (1.78:1) is generally used. However, for the photographing complying with the CinemaScope form having the aspect ratio of 2.35:1, there has been demanded an anamorphic converter for effectively utilizing pixels on an image pickup element to enhance image quality.
It is required for an anamorphic converter for the cinema that the suitable aspect ratio conversion is made, no eclipse is generated, an effective image surface of an imaging optical system can be sufficiently utilized, reduction in a quantity of marginal ray is less, and high optical performance is provided throughout zooming and focusing.
As described in Japanese Patent Application Laid-Open Nos. 2-13916, 6-82691, and Japanese Patent No. 2,817,074, the front converter type has advantages that a structure is simple, and an effective diameter is ensured irrespective of a conversion ratio to avoid generation of the eclipse. On the other hand, there is encountered a problem that the size is large, and a change in astigmatism due to focusing occurs.
In addition, as a technique for correction of astigmatism due to focusing, there were proposed the techniques described in Japanese Patent Application Laid-Open Nos. 48-24048, 3-25407, 5-188271 and 5-188272. In these techniques, however, there is encountered a problem that correction means within a converter must be driven in conjunction with focusing in an imaging optical system, and hence a complicated mechanism is required.
The rear converter type has an advantage that there occurs no change in astigmatism due to focusing. However, a problem arises that when a conversion magnification on a vertical side and a conversion magnification are not suitably set, the eclipse is generated and a field angle of an imaging optical system is changed so that an effective image surface can not be sufficiently utilized.
As the rear converter type, there are a system having no primary image formation as shown in FIG. 31, and a system having primary image formation as shown in FIG. 32.
In FIGS. 31 and 32, α1 is an emission inclination angle of axial marginal ray from an imaging optical system, and α2 and α3 are emission inclination angles of axial marginal ray from an anamorphic converter AC.
In case of the rear converter type having no primary image formation, as shown in FIG. 31, an axial marginal ray from the imaging optical system needs to be made nearly afocal with a negative lens. At the same time, since an off-axial chief ray is leapt up, an off-axial chief ray emission height hb2 from a converter final surface becomes large. Hence, a problem occurs that vignetting is increased to reduce a quantity of axial marginal ray, and an off-axial chief ray emission inclination angle αb2 is increased to shorten an exit pupil, and thus an influence of the color shading by a color separation optical system becomes easy to be generated.
The system having no primary image formation, i.e., the rear converter in which both focal length conversion magnifications βx and βy are positive values is proposed in JP 3,021,985 B. In this case, however, since the rear converter is prescribed so that a positive refracting power is obtained in a horizontal direction, and a negative refracting power is obtained in a vertical direction, the rear converter has an effect of lengthening a focal length in addition to an effect of converting an aspect ratio. As a result, there is a problem that a field angle becomes narrow, and if the field angle is tried to be ensured, then an image pickup means having a larger image size is required, and if the image pickup means having the larger image size is used, then an exit pupil position becomes relatively short so that an exit angle of an off-axial chief ray of a peripheral portion of a screen becomes large, and hence the shading or the like is generated.